Solid state sensors have been previously proposed by the art to measure the fugacity of various chemical species, such as, for instance, oxygen, sulfur and fluorine, in gases, liquids and solids at elevated temperatures. The oxygen fugacity sensor, sometimes simply called the oxygen sensor, is probably the most widely used of such solid state sensors. The foundation for the oxygen sensor was provided by K. Kiukkola and C. Wagner, J. Electrochem Soc. 104, 379 (1957). The use of the oxygen sensor for the rapid determination of oxygen in liquid steels, in the steel making process, is described in J. Metals, G. R. Fitterer, September, 92 (1967).
A general review of solid electrolyte fugacity sensors for oxygen and other gases is described in Research Techniques for High Pressure and High Temperature, M. Sato, edit. by G. C. Ulmer, Chapt 3, Springer-Verlag, New York 1971, p. 367.
Typically oxygen sensors consist of a ceramic solid state electrolyte (&gt;99% oxygen ion conductivity) such as zirconia doped with CaO or Y.sub.2 O.sub.3. Ceramics are fabricated as closed- or open-end tubes or as dense, thin discs. Opposed surfaces are metallized (e.g. with Pt, Au, Ag, etc.) and metal leadouts measure an open-circuit voltage according to, ##EQU1## Temperature (generally, T&gt;673 K), is measured independently, typically by thermocouples positioned adjacent to the sensing (presumably isothermal) portion of the cell either internally or externally. A reference, known f.sub.0.sbsb.2 [e.g. air (f.sub.0.sbsb.2 =0.21 atm) or oxygen (f.sub.0.sbsb.2 -1 atm)] is provided to one of the surfaces of the sensor. With T(K), f.sub.0.sbsb.2 (reference), and a measured voltage E, f.sub.0.sbsb.2 (unknown) can be calculated.
For many years researchers have used oxygen sensors not only to monitor and control the fugacity of oxygen in gas mixtures but to measure chemical equilibria associated with solid-liquid-gas redox reactions. Subsequently, industry has developed probes to control fuel/air combustion for large glassmaking furnaces, to monitor exhaust gases from automobiles, and to determine the oxygen content of molten metals in metallurgical processes. Numerous other industrial applications related to measurement and control of combustion processes will develop in the future.
Measurement of temperatures (T(K) in equation 1) necessary for laboratory and industrial measurements of oxygen fugacity has been generally limited to methods related to, (1) mechanical/expansion, (2) the Seebeck-Peltier effect (thermocouples), (3) electrical resistivity, and (4) optical pyrometry.
Recently, proposals have been made to utilize fluidic temperature sensing methods, based upon change of the fluid properties, such as viscosity, density, etc., with temperature. A tube fabricated from an inexpensive ceramic (Al.sub.2 O.sub.3, MgAl.sub.2 O.sub.4 (spinel), ZrO.sub.2) can be the temperature probe. The fluidic capillary thermometer operates on a very small flow of fluid (e.g., 10.sup.-6 m.sup.3 /sec for air) provided through an interior channel. At the "hot" or sensing end, the channel is reduced to a capillary flow resistance which can be described with the following relationships, EQU R=(.DELTA.P)/Q=C.mu.=f(T)
where
R=flow resistance PA1 .DELTA.P=pressure drop across capillary PA1 Q=volumetric flow rate PA1 .mu.=absolute viscosity PA1 T=temperature PA1 C=constant for a given probe
Resultant changes in pressure drop due to changes in viscosity, and hence, temperature, are then amplified to a usable level by inexpensive, low-noise, laminar fluidic pressure amplifiers. Ultimately, a pressure output directly as a function of temperature is provided.
To provide a local temperature measurement, the capillary is made very small and short, typically about 2 cm long and 0.5-0.75 mm in diameter. A fluidic circuit, incorporating biasing resistors and a fluidic amplifier all operating with the working fluid, is used to provide temperature output reading. The sensor is merely a variable resistor in a bridge network and is well known in the art as demonstrated in the literature by Drzewiecki, T. M. and Phillippi, R. M., "Fluidic Thermistors or Fluidic Temperature Sensing with Capillaries," Trans. ASME, Engineering for Power, Vol. 99, No. 3, July 1977 and Drzewiecki, T. M., Phillippi, R. M., and Paras, C. E., "Fluidics--A New Potential for Energy Conservation by Continuous High Temperature Monitoring and Control," Heat Transfer in Energy Conservation, ASME, 1977.
The output pressure differential is proportional to the temperature of the gas flowing through the capillary and may be read out on a pressure gage or with an electronic transducer. Typically with a gage, accuracy of .+-.1.degree. C. may be expected. Greater accuracy is obtainable with a transducer due to the ability to expand the scale. Readings to .+-.0.01.degree. C. would not be unreasonable with a transducer.